RESPIRATORY NERVE-CENTRE. 631 



ous glands etc., important points which are fully considered in their appro- 

 priate place. The various reflex centres in the medulla have been located 

 chiefly by a study of the relations of the . gray matter to the deep fibres of 

 origin of certain of the cranial nerves. The centre for the orbicularis oculi 

 muscle is related to the origin of the large root of the fifth nerve and the 

 origin of the facial ; and the integrity of these two nerves is necessary to the 

 reflex act of closure of the eyelids. The impression which produces the act 

 of sneezing is conveyed to the medulla through the nasal branch of the 

 fifth possibly sometimes through the olfactory nerves and excites certain 

 of the expiratory muscles. Impressions conveyed to the medulla by certain 

 sensory branches of the pneumogastrics give rise to the reflex acts of cough- 

 ing. The reflex acts of swallowing and vomiting also depend upon centres 

 in the medulla oblongata. There are centres, also, which influence the 

 glycogenic action of the liver, the secretion of saliva and the secretion of 

 sweat. The vaso-motor centres will be considered in connection with the 

 physiology of the vaso-motor nerves. The centres connected with respira- 

 tion are so important that they demand special description. 



Respiratory Nerve- Centre. In 1809, Legallois made a number of ex- 

 periments upon rabbits, cats and other animals, in which he showed that 

 respiration depends upon the medulla oblongata and not upon the brain ; 

 and he farther located the part which presides over the respiratory move- 

 ments, at the site of origin of the pneumogastric nerves. Flourens, in his 

 elaborate experiments upon the nerve-centres, extended the observations of 

 Legallois, and limited the respiratory centre in the rabbit, between the upper 

 border of the roots of the pneumogastrics and a plane situated about a quar- 

 ter of an inch (6'4 mm.) below the lowest point of origin of these nerves; 

 these limits, of course, varying with the size of the animal. Following these- 

 experiments, Longet has shown that the respiratory centre does not occupy 

 the whole of the medulla included between the two planes first indicated by 

 Flourens, but that it is confined to the gray matter of the lateral tracts, or 

 the intermediary fasciculi. This was demonstrated by the fact that respi- 

 ration persists in animals after division of the anterior pyramids and the 

 restiform bodies. Subsequently, Flourens restricted the limits of the respir- 

 atory centre and fully confirmed the observations of Longet. 



The portion of the medulla oblongata above indicated presides over the 

 movements of respiration and is the true respiratory nerve-centre. Nearly all 

 who have repeated the experiments of Flourens have found that the spinal 

 cord may be divided below the medulla oblongata, and that all of the en- 

 cephalic ganglia above may be removed, respiratory movements still persist- 

 ing. It is a very common thing in vivisections to kill an animal by break- 

 ing up the medulla. When this is done there are no struggles and no mani- 

 festations of the distress of asphyxia. The respiratory muscles simply cease 

 their action, and the animal loses instantly the sense of want of air. A 

 striking contrast to this is presented when the trachea is tied or when all 

 of the respiratory muscles are paralyzed without touching the medulla. 



The relations of the respiratory centre have already been fully considered 



